As noted by R. Kingslake in Lens Design Fundamentals, Academic Press, Inc., 1978, at page 322: "The classical two-mirror systems used in telescopes date from the seventeenth century. They were either of the Gregorian form with a concave parabolic primary mirror and a concave elliptical secondary, or of the Cassegrain form with the same parabolic primary but a convex hyperbolic secondary."
Historically, the telescope invented by Cassegrain in 1672 had a concave paraboloidal primary mirror and a convex hyperboloidal secondary mirror of shorter focal length than the primary mirror. The original telescope of Cassegrain was corrected for spherical aberration, but was not corrected for any of the off-axis geometrical aberrations. However, at the present time, the term "Cassegrain" is often used in a generic sense to designate any two-mirror telescope (i.e., any telescope having a primary mirror and a secondary mirror), regardless of the geometrical configurations of the mirrors. Even an afocal two-mirror telescope (i.e., a telescope for which the initial object distance and the final image distance are both infinite), as first described by Mersenne in 1636, is now often considered to be a special case of a Cassegrain-type telescope.
In 1905, Schwarzschild designed a telescope generally of the Cassegrain-type using oblate ellipsoidal primary and secondary mirrors arranged in a so-called "reverse telephoto" configuration, which provided correction for spherical aberration, coma, astigmatism and field curvature. A description of the Schwarzschild telescope is provided by D. Korsch in Reflective Optics, Academic Press, Inc., 1991, at page 169. Another variation on the original Cassegrain telescope design was developed in 1922 by Ritchey and Chretien, who used weakly hyperboloidal primary and secondary mirrors in order to achieve correction for coma. The Ritchey-Chretien telescope is discussed by R. Kingslake in Lens Design Fundamentals, Academic Press, Inc., 1978, at page 323.
It was noted by A. E. Conrady in an article entitled "Optical Systems with Non-spherical Surfaces", Monthly Notices of the Royal Astronomical Society, January 1920, page 320, that: "For the simple reason that the natural tendency of the abrading processes used in the finishing of optical elements is to produce true spherical surfaces and that deliberate departures from this form greatly increase the difficulty and cost of production, the use of `figured` [i.e., non-spherical] surfaces has been avoided as far as possible in the wholesale manufacture of optical instruments, the perfection of all of which depends largely, or more often absolutely, upon the skillful compensation of antagonistic spherical aberrations at successive strictly spherical surfaces." The formidable technical difficulties and the high cost involved in manufacturing accurate non-spherical conicoid mirror surfaces has stimulated efforts to develop designs for telescopes using spherical primary and secondary mirrors in combination with refractive elements (i.e., lenses) to obtain correction for chromatic aberration and for the various geometrical aberrations.
In 1932, Schmidt designed a telescope comprising a single mirror of spherical configuration and an aspheric lens (called a "corrector plate") positioned perpendicular to the optic axis of the mirror. The corrector plate of the Schmidt telescope functions like the refractive portion of a Mangin mirror in correcting for spherical aberration. The Schmidt telescope is well-corrected for astigmatism, since the corrector plate defines the aperture of the telescope at the center of curvature of the spherical mirror. The Schmidt telescope is also well-corrected for coma.
The problem of correcting for Petzval curvature in astronomical telescopy was discussed by J. G. Baker in an article entitled "A Family of Flat-Field Cameras, Equivalent in Performance to the Schmidt Camera", Proc of Amer. Phil. Soc., Vol. 82, No. 3, (April 1940), pages 339-349. At that time, the principal difficulty associated with Petzval curvature was the requirement that the photographic plates used in astronomical photography be distorted into spherical surfaces in order to provide high-resolution imaging. Analogous difficulties are now of concern in contemporary instrumentation using large focal plane arrays. Baker drew upon the aforementioned technique of Schwarzschild to produce a flat focal plane, but was unable to correct for astigmatism. The Baker article refers to work by Wright, which indicated that astigmatism cannot easily be eliminated by the use of refractive elements and corrector plates.
In 1941, Maksutov adapted the techniques of Schwarzschild and Schmidt in order to achieve correction for chromatic aberration, spherical aberration, coma and astigmatism in a telescope having concentrically arranged spherical primary and secondary mirrors. The Maksutov telescope comprises meniscus lens elements with spherical surfaces, which provide a small negative optical power. The meniscus lens elements of the Maksutov telescope function in combination with each other in the manner of a Schmidt corrector plate to correct for spherical aberration. A portion of the convex rear surface of one of the meniscus lens element of the Maksutov telescope can be silvered to function as the secondary mirror of the telescope in the manner of a telescope of Schwarzschild design. The aperture of the Maksutov telescope is ordinarily located at the common center of curvature of the primary and secondary mirrors. A description of the Maksutov telescope is found in an article by D. D. Maksutov entitled "New Catadioptric Meniscus Systems", J. Opt. Soc. Am., Vol. 34, No. 5, (May 1944), pages 270-284. The Maksutov telescope is not readily adaptable to a Mersenne-type afocal telescope.
The design of the Maksutov telescope is highly constrained, inasmuch as the primary and secondary mirrors must be concentric in order to achieve the intended corrections for spherical aberration, astigmatism and coma. Variations on the Maksutov telescope, which have been developed to provide better color correction and reduced spherical aberration, are discussed in detail by A. Bouwers in Achievements in Optics, Elsevier Publishing Company Inc., (1950), pages 16-45. Telescopes designed by Bouwers are described in U.S. Pat. Nos. 2,420,349; 2,492,461; and 2,748,658. However, the Bouwers telescopes are characterized by the same constraint as the Maksutov telescope, viz., that the spherical primary and secondary mirrors must be concentric in order to correct for coma and astigmatism. The requirement of concentricity for the primary and secondary mirrors is a very significant design constraint, which involves alignment sensitivities that adversely affect manufacturability and cost.
Houghton, in U.S. Pat. No. 2,350,112 granted in 1944, disclosed a single-mirror catadioptric telescope (i.e., not a Cassegrain-type telescope) in which substantially full correction for spherical aberration and coma is achieved by means of a corrector lens system (viz., a doublet) located beyond the focal plane of the single mirror (a spherical mirror) of the telescope. However, Houghton indicated that correction for astigmatism could not be achieved for such a telescope merely by adjusting the configuration and location of the corrector lens system. The Houghton telescope is not readily adaptable to a Mersenne-type afocal telescope.